The invention relates to a method for thread-stitching of book blocks, in particular, thread-stitching with blast air, including steps of storing a length of binding thread, tensioning the binding thread and thread-stitching a book block. The invention additionally relates to a corresponding apparatus for thread-stitching, including a thread tensioning device, wherein a binding thread can be stored and tensioned with a thread tensioning device. Further, the invention relates to a book production line equipped with such an apparatus.
A device for pulling a binding thread into a transfer section between a thread spool and a sewing needle is known from the European patent document EP 1155872 A1. With this device, which is embodied as a component of a thread-stitching machine, a binding thread needed for the stitching operation is first unwound from a spool and is then guided with the aid of different handling devices to a sewing needle and is then used to thread-stitch a book block. The handling devices comprise a plurality of conveying lines through which the binding thread is respectively conveyed, respectively with the aid of blast air. Two of these handling devices involve a thread-clamping device and a thread-tensioning device.
Furthermore known are devices for pulling a binding thread into a transfer section between a thread spool and a sewing needle which can operate without using blast air.
The applicant's own thread-stitching machine sold under the trade name Ventura comprises a thread-clamping device as well as a thread-tensioning device. The thread-clamping device includes two discs, wherein one of these discs is attached to a bolt while the other disc is positioned so that on the bolt it can be displaced in an axial direction with the aid of a spring. Between these discs, a through opening is provided in the bolt, at a right angle to the bolt axis, through which the binding thread is guided. The discs are pressed together with spring pressure, wherein the binding thread is correspondingly released or held in place by activating a pneumatic cylinder that is connected to the bolt.
The thread-tensioning device comprises a mechanically driven tubular shaft, which is positioned to rotate, through which several axially spaced-apart tubes are fitted while arranged at a right angle to a shaft axis. A binding thread is guided through each of these tubes. By rotating the tubular shaft, the pull on the binding threads increases proportionally to how much an angle between the tubes increases relative to a thread direction which is not deflected. With an opened thread-clamping device, the thread-tensioning device will store a specific length of the binding thread moving through a tube for the stitching process by unwinding the respective binding thread from the spool through a rotation of the tubular shaft. With a closed thread-clamping device, an already stitched book block is tensioned through a further rotation of the tubular shaft prior to the stitching operation. With an additional rotation of the tubular shaft while the thread-clamping device is closed, a just stitched sheet is tensioned against a growing book block.
In this or a similar manner, known apparatuses for producing thread-stitched book blocks successively produce thread-stitched book blocks composed of several parts during a cyclic operation. Within a cyclic operation assigned to a book block, the thread-stitching device operates with sewing cycles per se, wherein one sewing cycle comprises angles ranging from 0° to 360°.
The diagram shown in FIG. 4 illustrates the course of a thread tensioning operation during these sewing cycles. The thread tensioning follows from the course of a rotational angle, plotted on the y-axis above the cycle angle shown on the x-axis. This rotational angle course results from an actuation of the tubular shaft via a cam drive, not shown herein, which always has the same movement characteristic and can intervene only to a limited degree in case of deflections.
Overlapping the sewing cycles, a specific binding thread length for the following sewing cycle is stored in a cycle angle range from 300° to 90° when the thread-clamping device is open. Once the thread-clamping device closes at approximately 90°, it remains closed until the cycle angle reaches approximately 120°, whereupon a thread stitch carried out during the preceding cycle is tensioned further by rotating the tubular shaft at a rotational angle of approximately 25° to −40°, which is followed by the stitching operation for an additional stitch. Up to the cycle angle of 250°, the binding thread continues to be fed to the needle for a stitching movement by further rotating the tubular shaft, which is then following by a subsequent tensioning of the additional stitch up to approximately 300° cycle angle.
Resulting therefrom are characteristic curves for the known method according to FIG. 4 which show that of necessity a large amount of binding thread is stored with high thread tension. A maximum adjustment results in a characteristic curve drawn with short dashes while a minimum adjustment of a mechanical superimposed gear with cam control for the rotation of the tubular shaft results in a characteristic curve drawn with longer dashes. FIG. 4 shows that essentially only a parallel displacement of the respective characteristic curve takes place between the maximum and the minimum adjustment.
The assumption is that thread-stitching machines provided with the known thread-tensioning devices are used only for the processing of medium or large book editions for which all book blocks are identically composed.
When producing micro-editions of books, for example personal photo albums, each book block must be considered a unique item. For that reason, each individual book block must meet standard qualification criteria, for example relating to the thread tension for the binding threads which combine the individual parts of a book block. Accordingly, the requirements for such micro-editions are different with respect to the tensioning of the binding thread, meaning that more flexible characteristics are required when changing between different, successively following production orders, wherein these requirements can be met only insufficiently by the solutions offered in the prior art.
Notwithstanding the above, the use of servo drives is known for cases where an electronically controlled drive is needed which meets special requirements for the dynamic, the control range and/or the accuracy of a movement.
For the aforementioned application, however, an electronic control is not required and would be unnecessarily expensive since book blocks can be produced securely in a process requiring few and seldom needed adjustments of the thread-tensioning devices with the above-described, improved mechanical superimposed gears, which have been tested over long periods of time. Even during the changeover between different book editions, the known thread-tensioning devices are rarely adjusted.